1. Field of the Invention
The present invention relates to an optical transmission system using wavelength division multiplexing (WDM) and a WDM transmission device such as an optical node or optical repeater used in the optical transmission system and, more particularly, to a WDM transmission device having signal light monitoring functionality and a signal light monitoring method for the device.
2. Description of the Related Art
In WDM optical transmission systems using the technologies of multiplexing a plurality of optical signals of different wavelengths onto a single optical fiber, various add-drop multiplexers (ADM) and optical repeaters having signal light monitoring functionality have been proposed.
For example, Japanese Patent Application Unexamined Publication No. H11-055100 discloses an optical ADM having an optical-signal fault monitoring means which detects a fault from received optical signals at different wavelengths, which are obtained by wavelength-demultiplexing input WDM light.
Moreover, Japanese Patent Application Unexamined Publication No. 2001-168841 discloses a WDM optical amplifier which is provided with an optical splitting section disposed at either of the input or output stage of the optical amplifier, a monitor circuit which monitors different-wavelength lights obtained by demultiplexing WDM light on a wavelength basis, and a computation circuit which controls, based on the monitoring result, any one of a variable optical attenuator and gain flattening filter disposed at the output stage, whereby the gain profile of the output light is flattened in wavelength terms.
However, the WDM transmission devices as described above cannot effectively perform signal light monitoring when an abnormality (or fault) occurs in a route between the input port of the WDM transmission device and the input of the monitoring means. For example, in both of the cases where an optical signal actually does not exist and where an abnormality has occurred in the route, the optical signal cannot be detected and therefore the monitoring means issues the same alarm. In other words, the monitoring means cannot discriminate between causes of a fault when an optical signal actually does not exist and when an abnormality has occurred in the route. Hence, signal light monitoring with high precision cannot be realized.
Moreover, the WDM transmission devices as described above cannot monitor an optical signal for a fault when an abnormality occurs in the device's monitoring means itself. Alternatively, due to a monitoring malfunction, there may be a situation in which no alarm is issued when it should be, or, conversely, an alarm is issued when it should not be. Such a situation results in degraded reliability.